An X-ray CT apparatus is an apparatus in which an X-ray tube device and an X-ray detector are oppositely disposed to rotate around an object, irradiates X-rays from a plurality of rotation angle directions (views) to detect the X-ray transmitted through the object for each view, and generate a tomographic image of the object based on the detected projection data. In the recent years, an FFS (Flying Focal Spot) X-ray tube device having a function to irradiate X-rays to a plurality of spots by shifting an X-ray focus has been developed. In the FFS X-ray tube device, an X-ray focal spot can be shifted to a plurality of positions by electromagnetically moving a position of an electronic beam entering the anode (target). Hence, a plurality of projection data whose X-ray irradiation paths are different can be acquired from the same rotation angle direction (view), which can improve spatial resolution of the X-ray CT apparatus (the FFS method).
By the way, there is a problem that spatial resolution around the center of the entire effective field of view is improved while the spatial resolution deteriorates in the peripheral portion other than the central portion in an image reconstructed using the conventional FFS method. On the contrary to this, a BFFS (Balanced Flying Focus Spot) method is suggested in the patent literature 1, which homogenizes and improves the spatial resolution of the peripheral portion by setting an optimal focal movement distance based on the number of views to be scanned during one rotation (an angle difference between adjacent views) and a distance between the X-ray tube device and the rotational center.